1. Field of the Invention
The present invention relates to a method for synchronizing overlapping cells over the air, and more particularly, to a method for precisely synchronizing overlapping cells by considering a propagation delay between a transmitter and a receiver.
2. Description of the Related Art
Time Division Duplex (TDD) applies time-division multiplexing to separate outward and return signals. Likewise, Frequency Division Duplex (TDD) applies frequency-division multiplexing to separate outward and return signals. For both FDD and TDD systems, it is recommended that all Base Stations (BSs) are time synchronized with respect to a common network timing signal. In an event of loss of the network timing signal, BSs should continue to operate and should automatically resynchronize to the network timing signal when the network timing signal is recovered.
Self-Coexistence occurs when multiple wireless systems of the same type exist at the same time (i.e., coexistence). In the case of IEEE 802.22, Self-Coexistence means coexistence of multiple overlapping IEEE 802.22 cells.
Precise network time synchronization is essential in general TDD systems, especially for Self-Coexistence situations. For example, when two units of Customer Premises Equipment, CPE_a and CPE_b are located in an overlapping cell area between two BSs BS_a and BS_b, if the two BSs are not synchronized, an uplink transmission between CPE_a and BS_a could destroy the downlink receiving between CPE_b and BS_b. In addition, precise network time synchronization is required to support a soft-handoff, the mobility management, inter cell communications and locating. Moreover, in an IEEE 802.22 system, for reliable spectrum sensing, it is desirable for neighboring cells to be quiet at the same time in order to establish a system-wide quiet period. The more precisely the neighboring cells synchronize, the less guard time is needed in order to align system-wide quite period pr coexistence slots.
In IEEE 802.22, a method for synchronizing overlapping base stations over the air was disclosed. In this method, however, it was assumed that a propagation delay between two units of user equipment was negligible. This assumption, however, is not always legitimate. In fact, a 10 Km distance between two units of user equipment may result in about 33 μsec of propagation delay, which is certainly not negligible. Therefore, it is important to take the propagation delay into account when performing time synchronization.